Image Fundamentals; Human Visual System

Human eye

1

Eye’s receptors • Rods and Cones – Cones • Roughly 6-7 million • Concentrated in the fovea • Highly sensitive to color

– Rods • • • •

75-150 million Not involved in color vision Sensitive to low-level illumination Help give an overall picture of the field of view

# of rods or cones per mm^2

Receptor distribution 180 000

blind spot

rods

rods

90 000

cones 20 000

-80

0 Fovea

80

Degrees from fovea

2

“See” your blind-spot

Close your left eye and stare at the cross mark in the diagram with your right eye

Optics of the eye 15 X = 100 17 X = 2.55mm

15m

X

100 m

17mm

3

Human visual acuity Visual acuity of a normal eye is 1/60 of a degree

X

X 2

2

X’

D’

D X = D tan( 2

2

)

Visual acuity calculations X = size of object D = distance from object

X = D tan( 2

Thus, X = 2*D tan(1/120 degree) X = 2*D*0.00014544 X = 0.00029088 * D

2

) = 1/60

Human acuity

How far can we be from an 1mm object and still see it? 1mm 0.00029088

=D

D = 3.437 meters

4

Human sampling

fovea

Human sampling

5

Some Terms ! Light !

radiant energy that is capable of exciting the retina

Luminous Intensity “Luminous Flux per solid angle” Measured in Candela

Illuminance

“Luminous Flux incident on a surface”. Not to be confused with Luminance which considers “projected” area (direction). Measured in “Lux”

Luminous Flux “Light energy per time” Measured in Lumens

Luminance

“Luminous Intensity” per projected area in a given direction Measured in Candela per sq meter (a nit)

Some Terms Brightness Brightness is subjective attribute of light to which humans assign based on the response the perceive

Luminous Intensity “Luminous Flux per solid angle” Measured in Candela

(often confused with luminance) Illuminance

“Luminous Flux incident on a surface”. Not to be confused with Luminance which considers “projected” area (direction). Measured in “Lux”

Luminous Flux “Light energy per time” Measured in Lumens

Luminance

“Luminous Intensity” per projected area in a given direction Measured in Candela per sq meter (a nit)

6

Talking about “visual” effects • It is hard to quantify visual effects

– Measurable quantities vs. perceived effect – Luminance vs. Brightness • Depends on the conditions • Depends on the person!

• Note that there is a difference in

– Light, Intensity, Luminance, Brightness – But don’t get upset about it – People (including myself) will use these terms incorrectly

Human eye and brightness • Can distinguish around 64 shades of brightness [2**6] • BUT, it is very adaptive to different lighting conditions

7

Mach Band Effect Perceived Brightness changes around strong edges.

Interesting Facts about Human Visual System • Simultaneous Contrast

8

Visual Masking Threshold intensity increases at background with large non-uniform spatial, temporal changes.

Image representation origin

x

f(x,y) Y

9

Image representation origin x

•Discretized in both •Spatial coordinates •Brightness

f(x,y) Y Y Similar to a matrix in Linear Algebra Individual elements are called: image elements, picture elements, (image points) “pels”

pixels

Image model • f(x,y) = intensity

– intensity equal some response to luminance

• 0 < f(x,y) < +inf

10

Image model • Basic nature of f(x,y) has two components: – f(x,y) = i(x,y) * r(x,y)

• i(x,y) is illumination – 0